As far as I know, most of the Fox Giants are unsold books. The 1948 Almanac of Crime- which seems to be the earliest of the main run of Fox Giants, although there are a few odd earlier ones- had an additional 16 page signature at the start, which was common to all copies but was still reprints. That issue also seems to have been made from copies of the issues printed at the same time, rather than stripped and rebound issues, since the separate issues were not stapled themselves, just the book as a whole. (I've seen another copy at a con, the contents are NOT all the same 4 issues like I thought was possible).
In general, though, the Fox giants are NOT reprints- they're coverless books aggregated under a new cover.